1. Field of the invention
The invention relates to a heating plate with cavities, in which electrically heatable heating bodies and a heat exchange medium are disposed in order to transmit heat emitted by the heating bodies to the plate, in which the cavities are fitted with connecting means to permit circulation by the heat exchange medium.
2. The Prior Art
Such heating plates are used in laminating devices, in particular, as a means of producing photovoltaic modules or alternatively for producing plates joined by thermal adhesives or thermoplastic materials. In applications of this type, it is very important to obtain a uniform temperature across the entire surface of the heating plate, because even slight temperature differences can lead to faulty products.
A first type of known heating plates has integrated heating elements, which impart heat to the plate by heat conduction. This being the case, the temperature distribution along the heating elements can not be controlled. Furthermore, the heating elements have to fit in matching orifices of the plate as exactly as possible to guarantee efficient heat transmission. However, this can lead to tension due to heat expansion. Contact corrosion can occur if the heating plate and the heating elements are made from different materials. If heat-conducting pastes are used in order to improve heat transmission, these can dry out over time which in turn means that temperature differences can occur across the surface of the heating plate.
In a second type of known heating plates, cavities are provided, through which a heat exchange medium such as oil, for example, is circulated, and the heat exchange medium is heated externally to the heating plate. The heat exchange medium naturally cools as it flows through the cavities, which alone can lead to undesirable temperature differences at the surface of the heating plate.
A heating plate is known from German utility model DE 296 10 952 U1, in which electric heating elements are disposed in cavities of the heating plate and the cavities contain a circulating heat exchange medium. Since the cavities in this heating plate are separated by supporting walls with orifices for the heat exchange medium, these cavities are relatively large and a correspondingly large amount of heat exchange medium is needed. The orifices cause undesirable flow losses, which lead to a high drop in pressure. As a result of the orifices, the flow in the cavities is difficult to control and regions occur in which virtually no heat exchange medium flows. This can cause local cooling of the heat exchange medium or local over-heating in the region of the heating elements, which in turn leads to undesirable temperature differences at the surface of the heating plate. If oil is used as the heat exchange medium, it can become “baked” on the heating elements due to over-heating, which impairs the exchange of heat.